The present invention relates to acceleration transducers and more particularly to piezoresistive acceleration transducers which can be easily fabricated using microcircuit techniques.
Piezoresistive transducers are well known in the prior art and generally utilize a plurality of piezoresistive strain gauge elements integrally formed on a flexible diaphragm of semiconductive material rigidly retained about its periphery. The diaphragm typically comprises a crystalline silicon of one conductivity type with the strain gauge piezoresistive elements formed of opposite conducitivity type by diffusion or other appropriate process. The strain gauge piezoresistors are located in predetermined stress positions about the surface of the flexible diaphragm and detect forces produced mechanically by fluid pressure or other forces applied to the face of the diaphragm. The forces are detected when bending of the diaphragm causes stress which changes the resistance of the piezoresistors in the area of the strain gauges. Thus, as the diaphragm bends in response to pressure, acceleration or other mechanical force, the magnitude of that force can be measured by detecting the changes in the electrical characteristics of the piezoresistors.
A variety of devices have been constructed which employ the piezoresistive effect. It has been found that different constructions and configurations improve the accuracy and sensitivity of the transducers. In some instances, for example, the crystal axis of the silicon or other semiconductive diaphragm material is oriented in predetermined directions and the piezoresistors forming the strain gauges are aligned with respect to that orientation. In other instances the number and interconnection of the gauges is varied to obtain increased sensitivity and compensation for inaccuracies that result during strain detection.
Temperature effects have been of particular interest since changes in temperature affect the electrical characteristics of the gauges and reduce sensor accuracy. One proposed method of overcoming temperature effects and other inaccuracies has been to use multiple gauges in a bridge circuit. The bridge circuit is constructed so that the temperature effects cancel one another thereby resulting in a more accurate reading. While such techniques have been successful to some extent, the devices have been somewhat limited in their application and acceptability in certain environments. In addition, the success of such techniques requires each of the bridge resistors to be formed of the same material and subjected to similar thermal environments which do not vary due to the location of the gauges on the diaphragm.
In an attempt to overcome temperature problems, another technique proposed the use of heating elements to maintain the environment of the transducer at a predetermined temperature. Normally a heating resistor is controlled to provide a specific temperature in response to a sensing element which detects the temperature of the area surrounding the transducer. As the temperature changes, the output of the heating element is either increased or decreased to compensate for the required temperature variation. This technique, however, requires substantial power output and substantial power dissipation where wide ranges of temperature are encountered. In addition, the more complex structure required to provide temperature compensation over wide ranges reduces the versatility of such transducers.
In aircraft applications particularly, sensors have been subject to high mechanical stresses due to vibration and other operational conditions as well as to wide temperature variations with changes in altitude. While solid-state devices, including the above-mentioned piezoresistive configurations, are better able to withstand those stresses and temperature effects than prior mechanical devices, there is still a need to improve the construction and configuration of the transducer to improve accuracy and to limit breakage and damage. In particular, there is a need to prevent sensor damage during rough handling or high mechanical stresses such as are encountered during severe weather conditions. There is also a need to protect the transducer from damage caused by resonance when the structure is subject to the natural resonant frequencies of aircraft engines and the like. By controlling the configuration to minimize damage in such environments, the accuracy as well as the repair and replacement costs can be significantly improved.
In accordance with the above, the present invention has been developed to overcome the above known and other deficiencies and, more particularly, to provide an improved acceleration transducer for use in avionics systems.